One obvious piece of evidence is the inability of humans to behave in a completely moral fashion. Another obviouss piece of evidence is the observation that the younger a person is, the less capable they seem to be of acting in a moral fashion (children routinely act towards each other in the sandbox in ways that would result in criminal prosecutions among adults). Children have to be taught to be polite, obedient, respectful, generous, and so forth -- they don't do it naturally.
There is obviously some aspect of human nature that is not-good. Since sin is a general word for the not-good, and since it seems to be present from the origin of humans, "original sin" seems a reasonable term for this apect. If you have a better term than "original sin," I invite your input.
(This is one of the key aspects of the traditional "Protestant confirmation," as referenced by various books like Pilgrim's Progress. Awareness of sin does not come from studying the Bible, but from simply studying the human condition. The Bible doesn't teach about sin and evil -- the world does that. The Bible teaches about escape from sin and evil.)